The present invention relates to a brushless DC motor and a realization method of speed signal thereof, using a linear Hall-effect sensor which can measure the speed of a brushless DC (Direct current: referred as BLDC) motor without using a speed sensor so as to reduce the volume of a control system and cost. Specifically, the present invention uses a linear Hall-effect sensor instead of a conventional latch type Hall-effect sensor, for generating a commutation signal and speed signal of a BLDC motor. Therefore, it becomes possible to eliminate a separate speed sensor generally equipped in a conventional BLDC motor and naturally, to exclude the possibility of a sensor failure, thereby ensuring more space and higher reliability.
In many cases of servomotor operation, a speed sensor is used for detecting a speed signal. The problem of such type of design is that additional space is required for installing a motor and a speed sensor. However, in many cases, there is no enough space for installing such speed sensor, and otherwise, it increases cost owing to such installation of two separate parts. For solving the problems, some approaches to eliminate a speed sensor have been proposed. Typical approaches for acquiring speed information without using speed sensor are counting the pulse of latched Hall-effect sensor and using mathematical observer.
A method for counting a Hall-effect sensor signal, which is useful in a higher speed range where pulses periodically occur, can accept quantization error and dynamic range in measuring speed. In the meantime, this technique does not work well in a lower speed range. In a technical field regarding, for example, a position control system, a motor operates over a wide speed range from standstill to the maximum acceptable speed. Think of an extreme case in which a motor rotates at a low speed so as to generate only one pulse during a certain time period. When the speed is gradually raised until two pulses are observed, the estimated speed will be twice as much as the previous case. In this case, quantization error will be nearly 100%. The utilization of smoothing algorithm may be helpful, however, the fundamental problem cannot be overcome. Other techniques which can eliminate a speed sensor are based on an observer. However, since such techniques are established by a mathematical equation of an electric motor, they are very sensitively affected by parameters of such electric motor. Specifically, the methods are easily affected by non-linear elements such as dead zone, backlash or the like, which makes a stable speed signal measure difficult.
More specifically, although a tachometer, an encoder or the like are generally used for obtaining information on a speed signal of a motor, such sensors requires additional space for being linked to the motor, thereby being disadvantageous in terms of space efficiency. Particularly, for example, contact-type sensors could cause a system failure owing to corrosion between the contact areas. By such reason, elimination of such speed sensor has been demanded, and thus various related studies have been done as well as underway.
For instance, a method using an observer which can estimate the speed of a motor by using software, can be mentioned. However, such method has problems such as limitation on gain selection for determining dynamic characteristics of observation device errors and re-constructional errors in connection with unreliability at low frequency. Therefore, when it is applied to a control system such as a fin actuator of a guided missile, it gets easily affected by disturbances and parameter variations, thereby having limitations on achieving excellent system performance and stability. As other methods, as it has been described in the forgoing description, there are M mode, T mode, M/T mode or the like which use a pulse clock number of a Hall-effect sensor in order to obtain place information of a magnetic pole of BLDC motor. Since a new speed value is calculated only at the time of pulse generation, at high speed in which pulses are sufficiently occurred, this method can realize the rotation speed of a motor by counting the clock number by pulses generated over a certain period of time. However, it has a problem that when a motor rotates at low speed, there may be an area without pulse generation within a certain time set, causing a phase lag. It means that, since a Hall-effect sensor has a small number of pulse clocks, reliable speed information of a motor can be obtained at high speed, however at low speed, phase lag and wrong speed signals are obtained. Therefore, this method is not suitable for being applied to a system which is operated over a wide range of a motor speed.